MFV r328253: 8835 Speculative prefetch in ZFS not working for misaligned reads

MFV r328253: 8835 Speculative prefetch in ZFS not working for misaligned reads

illumos/illumos-gate@5cb8d943bc8513c6230589aad5a409d58b0297cb

https://www.illumos.org/issues/8835:
Sequential reads not aligned to block size are not detected by ZFS
prefetcher as sequential, killing prefetch and severely hurting
performance.  It is caused by dmu_zfetch() in case of misaligned
sequential accesses being called with overlap of one block.

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Allan Jude <allanjude@freebsd.org>
Approved by: Gordon Ross <gwr@nexenta.com>
Author: Alexander Motin <mav@FreeBSD.org>